Seismic Clusters and their Characteristics at the Arabian Sea Triple Junction: Supportive Evidences for Plate Margin Deformations

Abstract

The plate margin features defining the Arabian Sea Triple Junction (ASTJ) are: the Aden Ridge (AR), Sheba Ridge (SR) with their intervening Alula-Fartak Transform (AFT), Carlsberg Ridge (CR) and Owen Fracture Zone (OFZ). Exact nature of ASTJ is presently debated: whether it is RRF (ridge-ridge-fault) or RRR (ridge-ridge-ridge) type. A revised seismicity map for ASTJ is given here using data for a period little more than a century. "Point density spatial statistical criterion" is applied to short-listed 742 earthquakes (mb ≥ 4.3), 10 numbers of spatio-temporal seismic clusters are identified for ASTJ and its arms. Relocated hypocentres help better constraining the cluster identification wherever such data exist. Seismic clusters actually diagnose the most intense zones of strain accumulation due to far field as well as the local stress operating at ASTJ. An earthquake swarm emanating from a prominent seismic cluster below SR provides an opportunity to investigate the pore pressure diffusion process (due to the active source) by means of "r-t plot". Stress and faulting pattern in the active zones are deduced from 43 CMT solutions. While normal or lateral faulting is characteristic for these arms, an anomalous thrust earthquake occurs in the triangular 'Wheatley Deep' deformation zone proximal to ASTJ. The latter appears to have formed due to a shift of the deformational front from OFZ towards a transform that offsets SR. Though ASTJ is still in the process of evolution, available data favour that this RRF triple junction may eventually be converted to a more stable RRR type.

Incipient status of dyke intrusion in top crust – evidences from the Al-Ays 2009 earthquake swarm, Harrat Lunayyir, SW Saudi Arabia

“…The earthquake points in the r-t plot with modelled diffusivity value D of 0.1 m 2 /s lie below the modelled parabolic curve (figure 6), where, the swarm relates to the diffusion of pore pressure in a poro-elastic fluid saturated medium. The pore-pressure perturbations that generate earthquake swarm in divergent pull-apart basins are initiated by magmatic dyke intrusion from mantle source (also refer Mukhopadhyay et al 2010Mukhopadhyay et al , 2011. A very high pore pressure activity with low diffusivity along narrow zone of volcanic dyke intrusion with depths from 15 to 20 km can be inferred.…”